A hollow structure which is used as a boom, an arm, or the like for making up a working mechanism of a hydraulic excavator is formed as a rigid box-shaped structure having a quadrangular cross-sectional shape by, for example, combining and welding two T-joints.
In this case, the T-joint is generally formed by performing fillet welding between a groove formed on a leading end side of an upright plate and the top surface of a lower plate in a state in which the upright plate is disposed on the top surface side of the lower plate in a T-shape.
Here, in forming the T-joint in the above-described manner, in a case where fillet welding is performed from one side (front surface side) in the thickness wise direction of the upright plate, the front surface side of the upright plate and the lower plate can be sufficiently welded. However, in the case where fillet welding is performed from the front surface side of the upright plate, unwelded portions can possibly remain between the back surface side of the upright plate and the lower plate, and therefore there is a drawback in that the strength of the T-joint disadvantageously declines.
In contrast, a welding method has been proposed in which a bead (penetration bead) is formed in advance on the top surface of the lower plate, while a groove is formed in advance on the front surface side of the upright plate, and then in a state in which the back surface of the upright plate is butted against the penetration bead of the lower plate, or in a state in which the upright plate is disposed in spaced-apart relation to the penetration bead, arc welding or the like is performed between the lower plate and the upright plate from the front surface side of the groove (e.g., Patent Literature 1: Japanese Patent Laid-Open No. H4-238670 A and Patent Literature 2: Japanese Patent Laid-Open No. H6-23544 A).
However, in accordance with the conventional welding method, in a preliminary step for the operation of disposing the upright plate to be welded on the top surface side of the lower plate, a penetration bead needs to be formed in advance on the top surface of the lower plate. Accordingly, in the welding method in accordance with the conventional art, a welding operation is performed in which a penetration bead is formed on the top surface of the lower plate (first welding step). Next, an operation of disposing the upright plate on the top surface side of the lower plate along this penetration bead is performed (plate assembling step). Further, after the plate assembling step, a welding operation is performed in which a welding bead is formed by fusing the penetration bead formed on the lower plate and the back surface side of the groove provided on the upright plate (second welding step).
In this case, the first welding step, the plate assembling step and the second welding step are generally performed in respectively different workplaces. For this reason, in the case where the first welding step, the plate assembling step and the second welding step are performed as in the prior art, it is necessary to move back and forth among different workplaces a number of times, so that there is a problem in that workability at the time of forming a T-joint disadvantageously declines.
Further, with the welding method in accordance with the conventional art, the shape of the welding bead formed in the second welding step is determined in correspondence with the position of the upright plate with respect to the penetration bead formed in advance on the lower plate. For this reason, in the plate assembling step, it is necessary to precisely effect the positioning of the upright plate with respect to the penetration bead formed in advance on the lower plate, and there is a problem in that the workability of this positioning operation has a tendency to deteriorate.